Large body size in the pygmy swordtail Xiphophorus pygmaeus

Abstract

Fishes in the genus Xiphophorus (swordtails and platyfishes) are well known for the influence of the pituitary ( P) locus on variation of male size at maturity both within and among species. We report the discovery of large male size ( >29 mm SL) in several populations of the swordtail X. pygmaeus, a species previously thought to consist of only small males (<29 mm SL). Large size is geographically restricted, and average male size varies significantly by site and year sampled in a pattern suggesting a recent origin and slow spreading of the large male phenotype. However, large male size is not strongly patternally inherited in this species, as it is in its two closest relatives, X. nigrensis and X. multilineatus, showing that large size does not result from the same genetic ( P locus) mechanism. Large X. pygmaeus males do not court, can exhibit the gold morph, do not possess swords and have slender body shape. In these traits they resemble small conspecific males and small males of X. nigrensis and X. multilineatus rather than large males of these latter two species. This shows that correlations between morphological and behavioural traits that occur in X. nigrensis and X. multilineatus are absent in X. pygmaeus.